1. Field of the Invention
The present invention is related to the field of electron emitters and more particularly, relates to a method for making stable, electron emitters and devices therefrom in diamond using an ion implantation technique.
2. Background of the Invention
There has been a great deal of research with respect to the physics of, and improved methods for fabricating, stable, modulatable electron (field) emitters (a type of cathode) having high current densities. Electron emitters are commonly used in such devices as power switches, microwave amplifiers, traveling wave tubes and the like.
Electron emitters emit electrons from structures at their ends, commonly referred to as xe2x80x9ctips.xe2x80x9d The xe2x80x9ctipsxe2x80x9d of emitters have a very small radius of curvature i.e. the tips are very pointed. The application of relatively small voltages in close proximity to an emitter extracts disproportionately large electron flows from its tip because the small radius of curvature of a tip concentrates the electric field.
The electric field extracts electrons from the conduction band and/or the valence band and/or Fermi level of the emitter material.
Devices of this type using tips and gates are commonly referred to as Spindt cathodes. A Spindt cathode is usually comprised of a micron-size cone or other structure with a sharp tip at the apex that is centered in a small-diameter hole. The size of the cone or other structure can vary. Furthermore, the shape of the cone can vary, as long as the structure contains a sharp tip. The cone is usually electrically conducting. Typically there is an electrically conducting film at or near the top of the cathode, usually in the shape of an annulus centered around the tip. The electrically conducting film may be used to apply an electrical potential near the tip of the cone and is called a xe2x80x9cgatexe2x80x9d. The tip of the cone typically lies in or below the plane of the xe2x80x9cgatexe2x80x9d and is centered in a hole in the gate. When the cone has a sharp tip, an applied voltage between the cone and the gate causes electrons to emit from the cone tip into the surrounding media (typically a vacuum) and to be collected by a third electrical conductor, the anode. The design of the Spindt cathode allows a small applied voltage between the gate and the tip over a sub-micron distance to extract a comparatively large amount of electrons. Spindt cathodes are typically fabricated in large arrays. Spindt cathodes are typically treated with a material having a low work function such as Cs. This treatment of the cathodes lowers the work function of the cathodes, thereby facilitating emission of the electrons.
Spindt cathodes often suffer tip instabilities. These instabilities are brought about by processes such as heating of the cathode, electromigration and ion sputtering from the gas phase. Ion sputtering occurs as the electrons ejected from the tip ionize background gas molecules near the tip. The ionized gas molecules are accelerated back towards the substrate containing the tip by the same electric field used to extract the electrons from the tip. The momentum damage from the ion colliding with the tip sputters and blunts the tip of each electron emitter. As the tip is blunted, the radius of curvature of the tip increases. This lowers the enhancement of the electric field at the tip. Furthermore, such processes change the surface composition of the tip by furthering undesirable processes at the tip such as oxidation. This can increase the work function of the tip and lower the electron emission for a given applied voltage.
Another problem inhibiting development of arrays of these devices is making uniform the voltage applied to, and current extracted from, the individual tips. These non-uniformity problems often occur because of variations in the morphology, form and structure of individual tips. These problems can also occur because of differences in the distance between the individual tips and their gates, and because of variations of the effective work function of the individual tips due to differences in surface chemistry. The results of this lack of uniformity among the individual tips within the array are most commonly: poor overall emission from the array, or emission of most of the current from only a small number of tips in the array. Emission from a small number of tips leaves the tips which are emitting most of the current prone to overheating and to catastrophic failure.
To address these problems, diamond has received much attention as an emitter surface because under some conditions diamond has negative electron affinity. Because of this negative electron affinity, the vacuum level falls below the conduction band minimum, and an electron in the conduction band encounters little or no energetic barrier to emission into the vacuum. However, for diamond to work here, the electrons must be transmitted thru the diamond. Further, in some applications the electrons must be able to move through the diamond lattice from the point of injection to the front surface of the lattice, and then cross the front surface/vacuum interface and exit the interface into the vacuum or be collected by a conducting film on the surface.
Because of these electron transport problems, it would be beneficial to minimize the thickness of the lattice through which the electrons move and to create a lattice (or material) which causes minimal or no energy loss to the electrons as they move through the lattice. Further, it is desirable to minimize the work function of the emitting surface, and to minimize any energy losses that occur at the surface during emission.
There have been attempts in the prior art to address some of the issues discussed above. Prior art patents of interest includes U.S. Pat. No. 5,990,604 to Geis et al.; U.S. Pat. No. 5,945,778 to Jaskie; U.S. Pat. No. 5,857,882 to Pam et al.; U.S. Pat. No. 5,757,344 to Miyata et al.; U.S. Pat. No. 5,670,788 to Geis; U.S. Pat. No. 5,258,685 to Jaskie et al.; U.S. Pat. No. 5,202,571 to Hirabayashi et al.; U.S. Pat. No. 5,141,460 to Jaskie et al.; and U.S. Pat. No. 5,129,850 to Kane et al.
The Geis et al. (""604) patent discloses a field emitter of wide-bandgap materials composed of a doped diamond film emitter formed by chemical vapor deposition combined with a metal compound through annealing, eching or ion bombardment. The Jaskie (""778) patent discloses an enhanced electron emitter composed of a diamond bond structure with an electrically active defect at the emission site which is said to act like a very thin election emitter with a very low work function and improved current characteristics. The Pam et al. patent discloses a method for the processing of materials for uniform field emission. The field emitters are composed of a polycrystalline film on a substrate formed by carbon ion implantation, annealing and then conditioning by scanning with an electrode. The Miyata et al. patent discloses a cold cathode emitter element composed of a diamond film emitter and a diamond insulating film. The Geis (""788) patent discloses a diamond cold cathode composed of a carbon ion implanted n-type conductivity diamond region and a doped homoepitaxial p-type conductivity diamond region with a junction between. The Jaskie et al. (""685) patent discloses a field emission electron source employing a diamond coating grown from carbon nucleation sites selectively disposed on a selectively shaped substrate. The Hirabayashi et al. patent discloses an electron emitting device with a diamond emitter layer formed on a semiconductor substrate. The Jaskie et al. (""460) patent discloses a method of making a field emission electron source employing a diamond coating. The diamond coated emitter is formed by ion implantation creating nucleation sites for diamond formation. The Kane et al. patent also discloses a method of making field emitters with a diamond coating disposed on a conductive or semiconductive material, wherein the field emitters are formed by ion implantation of carbon into a selectively shaped substrate to facilitate diamond formation, depositing the conductive layer, and then removing the substrate.
In accordance with the invention, a method for fabricating an electron emitter is provided. In one embodiment, the method comprises the steps of: implanting one or more ions into the surface of a diamond substrate including a diamond lattice including sp3 bonded carbon so as to damage the diamond lattice and to change at least a portion of the sp3 bonded carbon in the diamond lattice to a mixture of sp3 bonded carbon and sp2 bonded carbon, the damage to the diamond lattice material forming one or more emitter tips at or near the surface of the diamond lattice material tapering to a corresponding number of wider bases within the diamond lattice material; providing electrical contact to the base of the emitter tips; possibly growing one or more additional advantageous layers on the diamond substrate; and providing at least one gate on or under the surface of the diamond lattice or other additional advantageous layers.
Preferably, at least one gate is deposited or otherwise created on or below the surface of the protective layer.
Advantageously, at least one gate is deposited or otherwise created on or below the surface of the diamond substrate material before the protective layer is grown, or the gate can be subsurface.
Preferably, a mask having one or more holes therein is placed or formed over the surface of the diamond lattice material prior to implantation; and the mask may be removed after implantation.
Advantageously, the surface of the diamond substrate is terminated so as to lower the work function of the surface. Preferably, the surface is chemically modified with moieties containing one or more of the following: hydrogen, cesium, oxygen, fluorine, potassium, rubidium, lithium or any other alkali metal.
Preferably, at least a portion of carbon from the damaged area resulting from the implantation is electro chemically or otherwise etched out so as to leave a hollow mold formed from etched out portions of the damaged area, and the mold so formed is filled with a conductive filler material. Prior patents of interest include: U.S. Pat. No. 5,269,890 to Marchywka; U.S. Pat. No. 5,587,210 to Marchywka et al., U.S. Pat. No. 5,702,586 to Pehrsson et al.
Advantageously, the filler material comprises a metal, amorphous carbon, or silicon.
Preferably, the protective layer comprises a thin homoepitaxial layer.
In another embodiment, a conductive material is deposited or otherwise created on or under the thin protective layer so as to provide a gate structure. Advantageously, the conductive material is amorphous carbon.
In yet another embodiment, the method comprises the steps of: implanting one or more ions into the surface of a doped diamond substrate including a diamond lattice including sp3 bonded carbon, so as to damage the diamond lattice and to change at least a portion of the sp3 bonded carbon in the diamond lattice to a mixture of sp3 bonded carbon and sp2 bonded carbon, the damage to the diamond lattice material forming one or more tips at or near the surface of the diamond lattice material tapering outwardly to a continuous, electrically conducting base within the diamond lattice material; providing an electrical contact to the implanted region; and if necessary, providing a diamond layer or layers on the diamond substrate; and providing at least one gate.
Preferably, the diamond substrate is doped with nitrogen.
Advantageously, the upper protective layer can be doped with boron.
Advantageously, the thin protective layer or layers are comprised of one or more homoepitaxial diamond layers, either doped or undoped.
In a further embodiment, the method comprises the steps of: implanting one or more ions into the surface of a diamond substrate including a diamond lattice including sp3 carbon, so as to damage the diamond lattice and to change at least a portion the sp3 bonded carbon in the diamond lattice to a mixture of sp3 bonded carbon and sp2 bonded carbon, the damage to the diamond lattice material forming one or more implantation areas at or near the surface of the diamond lattice material tapering outwardly to a single base within the diamond lattice material; providing an electrical contact to the implanted regions; possibly providing an additional diamond layer or layers on the diamond substrate; implanting one or more ions into the surface of the diamond layer including a diamond lattice including sp3 bonded carbon so as to damage the diamond lattice and to change at least a portion of the sp3 bonded carbon in the diamond lattice to a mixture of sp3 bonded carbon and sp2 bonded carbon, the damage to the diamond lattice material forming one or more tips at or near the surface of the diamond lattice material tapering to a corresponding plurality of wider bases within the diamond lattice material, wherein the plurality of wider bases are in conductive contact with the continuous, electrically conducting base within the diamond substrate; possibly growing a thin protective layer on the diamond layer; providing at least one gate; and providing at least one electrical contact to the implanted regions, and extending from the conductive base layer.
Advantageously, a mask is used over at least a portion of the surface of the diamond layer before implantation.
The arrays of electron emitters fabricated from the method of the invention are stable, modulatable, and have a high current density. The devices can operate in either a continuous (DC) or a pulsed mode. Further, the field emitting arrays can be fabricated using a group of standard techniques such as lithography, implantation, patterning and etching.
Other features and advantages of the invention will be set forth in, or will be apparent from, the detailed description of preferred embodiments which follows.
Advantageously, conductive or metallic material is deposited or otherwise created on or under the surface of the diamond or any of its protective layers. The resulting structure acts as an anode for the capture of electrons emitted from the tips. Its performance in this regard may be enhanced by applying a positive voltage with respect to the tip(s) or gate(s).